Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device.
A common element in all of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device requires electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an implantable medical device can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes single cell batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer implantable medical devices. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need to explant and re-implant the implantable medical device or a portion of the device. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin.
Several systems and methods have been used for transcutaneously inductively recharging a rechargeable used in an implantable medical device, including U.S. Pat. No. 5,411,537, Munshi et al, Rechargeable Biomedical Battery Powered Devices With Recharging and Control System Therefor, (Intermedics, Inc.); U.S. Pat. No. 5,690,693, Wang et al, Transcutaneous Energy Transmission Circuit For Implantable Medical Device, (Sulzer Intermedics Inc.); U.S. Pat. No. 5,733,313, Barreras, Sr., FR Coupled Implantable Medical Device With Rechargeable Back-Up Power Source, (Exonix Corporation); U.S. Pat. No. 6,308,101, Faltys et al, Fully Implantable Cochlear Implant System, (Advanced Bionics Corporation); U.S. Pat. No. 6,324,430, Zarinetchi et al, Magnetic Shield For Primary Coil of Transcutaneous Energy Transfer Device, (Abiomed, Inc.); U.S. Pat. No. 6,516,227, Meadows et al, Rechargeable Spinal Cord Stimulator System, (Advanced Bionics Corporation); U.S. Pat. No. 6,505,077, Kast et al, Implantable Medical Device With External Recharging Coil Electrical Connection, (Medtronic, Inc.); European Patent Application 1,048,324, Schallhorn, Medical Li+ Rechargeable Powered Implantable Stimulator, (Medtronic, Inc.); PCT Patent Application No. WO 01/83029 A1, Torgerson et al, Battery Recharge Management For an Implantable Medical Device, (Medtronic, Inc.); and PCT Patent Application No. WO 01/97908 A2, Jimenez et al, An Implantable Medical Device With Recharging Coil Magnetic Shield, (Medtronic, Inc.).
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the implanted medical device. The external coil, or primary coil, is associated with the external power source or external charger, or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. This current can then be used to power the implanted medical device or to charge, or recharge, an internal power source, or a combination of the two.
For implanted medical devices, the efficiency at which energy is transcutaneously transferred is crucial. First, the inductive coupling, while inductively inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer which can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging, or recharging, is occurring the patient necessarily has an external encumbrance attached to their body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging, or recharging, can be accomplished limiting the inconvenience to the patient. Third, amount of charging, or recharging, can be limited by the amount of time required for charging, or recharging. Since the patient is typically inconvenienced during such charging, or recharging, there is a practical limit on the amount of time during which charging, or recharging, should occur. Hence, the size of the internal power source can be effectively limited by the amount of energy which can be transferred within the amount of charging time. The higher the efficiency of the energy transfer system, the greater amount of energy which can be transferred and, hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.
Differing attempts have been made to limit heat build-up during transcutaneous energy transfer involving implantable medical devices.
U.S. Pat. No. 6,324,431, Zarinetchi et al, Transcutaneous Energy Transfer Device With Magnetic Field Protected Components in Secondary Coil, (Abiomed, Inc.) and the similar U.S. Pat. No. 6,496,733, Zarinetchi et al, Transcutaneous Energy Transfer Device with Magnetic Field Protected Components in Secondary Coil, (Abiomed, Inc.) include an implantable medical device having a cage formed of a high permeability material in which the electronic components are mounted, which cage guides the flux around the components to prevent heating thereof.
U.S. Pat. No. 5,702,431, Wang et al, Enhanced Transcutaneous Recharging System For Battery Power Implantable Medical Device, (Sulzer Intermedics) and corresponding International Patent Application No. WO 98/11942A1, Wang et al, Enhanced Transcutaneous Recharging System For Battery Power Implantable Medical Device, (Sulzer Intermedics) reduces instantaneous charging current or duty cycle and maximizes transcutaneous energy transmission at different current levels by placing different capacitors into the circuit depending upon the magnitude of the charging current.
U.S. Patent Application No. 2004/0015198A1, Skarstad et al, Method and Apparatus For Reducing Heat Flow, (Medtronic) and the corresponding International Patent Application No. WO 2004/009179A1, Method and Apparatus For Reducing Heat Flow, (Medtronic) include at least one sacrificial member positioned in the casing of an implantable medical device to absorb at least a portion of a heat flow in the volume.
U.S. Pat. No. 6,586,912, Tsukamoto et al, Method and Apparatus For Amplitude Limiting Battery Temperature Spikes, (Quallion LLC) employs a heat absorber closely thermally coupled to the battery of an implantable medical device.
U.S. Pat. No. 5,991,665, Wang et al, Self-Cooling Transcutaneous Energy Transfer System For Battery Powered Implantable Device, (Sulzer Intermedics Inc.) discloses a self-cooling transcutaneous energy transfer system for providing power to an implantable medical device. A cooling fan is attached to a housing supported above the human body by a base so as to define a space between the housing and the body.
Prior art implantable medical devices, external power sources, systems and methods have not always provided the best possible control or limitation of temperature build-up while maintaining effective transcutaneous energy transfer and/or charging.